Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems, there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems, the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems, the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems, the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are near the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing method is the Frequency Division Duplexing (FDD) as shown in FIG. 2(A). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is the Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half; the RF channel may be used for UL. In some communication systems, the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically. In some communication systems, a predefined set of configurations may be used to select between different DL and UL duration ratios as shown in FIG. 2(B) with two different configurations. These predefined configurations are referred herein as TDD configurations.
Yet another commonly used duplexing method is the Half-duplex FDD (H-FDD) as shown in FIG. 2(C). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation.
The TDD duplexing method has the advantage of enabling simpler implementation of the RF and baseband components of the client terminal since either only transmit or only receive may be active at any given time.
For the client terminal to be able to successfully receive and transmit signals in DL and UL directions respectively, it is essential for the client terminal to know the timing of the signals in both DL and UL directions. The timing for the signals from the client terminals to the base station may depend on various factors, but primarily on the position of the client terminals with respect to the base station. Most wireless communications systems broadcast some type of beacon signal to help the client terminals synchronize to the network. A client terminal may use this beacon signal to establish time and frequency synchronization with the base station.
In a TDD wireless communication system, the timing of the signal from client terminals to the base station (UL) may further depend on the boundary between the DL and UL at the base station. In TDD wireless communication systems, the base station generally broadcasts system information about the starting time of the UL direction. Client terminals may receive the system information about the DL and UL switching point and other information before attempting communication in the UL direction.
In many wireless communication application scenarios, the users may be more likely to be in indoor environments such as in homes and offices. Users may be in outdoor environments when going from one indoor environment to another indoor environment, for example going from home to office and vice a versa. However, in most wireless communication systems there is a major problem with the coverage of indoor environments because of signal loss during penetration of the signals through walls and other structures. In the newly emerging Machine Type Communication (MTC) applications, the machine type devices may be located deep inside building structures where the signal from base station may experience very high penetration loss. To enable MTC applications it may be essential to improve indoor coverage for reliable communication. Therefore, it may be important to find methods and apparatuses that can improve the signal coverage for indoor environments for reliable communication.
Different approaches may be used to address the signal coverage issues. One approach may be to have layers of base stations where one layer of base stations with high power and tall antenna towers covering a large area and another layer with lower power nodes with possibly lower height antennas may be installed on the exterior or interior of office buildings and houses. The base stations with higher power are normally referred as macro cells. The lower power nodes in the second layer may include nodes with different capabilities such as femto cells, Relay Stations, Remote Radio Heads, etc.
The use of repeaters may be another approach to improve coverage and performance. Repeaters are typically mounted in an advantageous position for signal reception from the base station. This may be either on the exterior of the building or a house or it may be interior to the building at a location where the signal strength and quality may be better. A repeater receives the signal from a base station, boosts it and then retransmits it to the interior of the building or a house. Similarly, the signals from the client terminals inside the building are received by the repeater, which are then boosted and transmitted to a base station. In general, the repeater may be transparent to the client terminals that are near the repeater even though they may be sending and receiving the information through the repeater.
Different types of repeaters are used in practice. For example, On-Frequency Repeater (OFR) where the boosted signal is retransmitted on the same channel as that of the original channel from the base station. Another example is Frequency Shifting Repeater (FSR) or Frequency Translating Repeater (FTR) where the boosted signal is retransmitted on a channel that is different from the original channel used by the base station.
Repeaters may have the advantage that they are much less complex compared to a conventional base station in a femto cell. Furthermore, repeaters may not require use of any additional RF channels from the network operator (although some repeater implementations may use other RF channels from the network operator's licensed frequency bands or may use unlicensed frequency bands such as Industrial, Scientific and Medical (ISM) radio band or Unlicensed National Information Infrastructure (UNII) radio band). This may eliminate any frequency resource coordination and planning requirement for the network operator. As a result, repeaters may be lower cost and a preferred alternative in many scenarios to address the coverage issues. A repeater may determine the timing of the UL portion by first receiving the system information in the DL that describes the starting time of the UL direction. However, this may require the repeater to implement a complete RF and baseband receiver or a major part of it. This may normally lead to increased cost.
The entity that may be used to enhance the coverage and performance such as repeater, smart repeater, OFR, FSR, FTR, remote radio head, relay station, etc., is referred herein as Signal Enhancer.
A method and apparatus are described that enable the Signal Enhancer to determine the starting time of the UL direction in a TDD wireless communication system without implementing a complete or major portion of a receiver. This results in a lower cost solution for improving the coverage and the overall communication system performance.